1. Field of the Invention
The present invention concerns an electrochemical generator comprising a grafted microporous separator.
2. Description of the Prior Art
The positive and negative electrodes in an electrochemical generator are separated by an insulating material which allows ionic conduction whilst avoiding any electrical contact between the two electrodes. These properties must be maintained throughout the useful life of the generator. In order to maintain electrical insulation between the two electrodes, the separator must be mechanically and chemically stable under the conditions of use. In addition, high ionic conductivity requires that the separator is uniformly wetted by the electrolyte. Finally, using such a separator on an industrial production line requires mechanical strength so that it can be manipulated easily and stand up to successive operations.
For lithium anode rechargeable generators, the separator is constituted by a microporous polymeric film, specifically a polyethylene or polypropylene film. These films are sufficiently stable with the electrolytes in normal use, but are difficult to wet.
One solution consists in introducing an additive into the electrolyte. A further possibility lies in adding a wetting agent to the surface. These substances are not perfectly inert and their degradation reduces the life of the generator.
Researchers have therefore turned their attention to grafted separators. The monomers which are grafted are olefinic compounds, mainly acrylic and methacrylic acids, vinylpyridine, vinylpyrrolidone, .tau.-butyrolactone (EP-A-0 262 846), and other substituted vinyl derivatives (U.S. Pat. No. 4,376,168 and U.S. Pat. No. 4,414,090).
Grafting may be effected in the presence of a peroxide or azo catalyst (U.S. Pat. No. 4,287,275) or a metallic catalyst: colloidal silver, platinum, gold, nickel or copper (U.S. Pat. No. 3,376,168).
More efficiently, ionizing radiation may be used, such as gamma rays from a cobalt-60 source (U.S. Pat. No. 4,414,090), an electron beam (U.S. Pat. No. 4,283,442), ultraviolet radiation, plasma emission, or ozonization (U.S. Pat. No. 4,206,275). Irradiation and grafting may be carried out simultaneously or in two stages (U.S. Pat. No. 4,283,442).
The improvement in properties in grafted separators is, however, only observable for electrolytes with a low salt concentration (0.8M LiAsF.sub.6 : EP-A-0 262 846).
The object of the present invention is to provide a grafted copolymer separator which is wettable in electrolytes having a high salt content.
A further object of the present invention is to produce a grafted separator having improved properties as well as mechanical strength and production conditions which enable it to be used in the industrial manufacture of electrochemical generators.